Relative density

Definition and Measurement of Relative Density

• Relative density is the ratio of the density of a substance to the density of a reference material.
• It is a dimensionless quantity.
• Specific gravity is another term used interchangeably with relative density.
• Relative density for liquids is typically measured with respect to water at its densest.
• For gases, the reference is air at room temperature.
• Relative density is calculated by dividing the density of the substance by the density of the reference material.
• The reference material is often assumed to be water at 4°C.
• In SI units, the density of water is approximately 1000 kg/m³ or 1 g/cm³.
• The relative density of gases is often measured with respect to dry air at 20°C and 101.325 kPa.
• The molar mass of the gas can be used to calculate relative density with respect to air.

Relationship to Buoyancy

• If a substance's relative density is less than 1, it is less dense than the reference material and will float in it.
• If the relative density is greater than 1, the substance is denser than the reference material and will sink.
• An ice cube, with a relative density of about 0.91, will float in water.
• The relative density of a substance determines its buoyancy in a given medium.
• Temperature and pressure must be specified for both the sample and the reference when considering buoyancy.

Applications in Industry

• Specific gravity is commonly used in industry to determine the concentration of solutions.
• It provides information about the concentration of materials such as brines, syrups, juices, acids, etc.
• Specific gravity is particularly convenient for calculations when the density of water is divided by 1000 or 1, depending on the units.
• It is used in various industries, including brewing, to measure the concentration of substances.
• Specific gravity is a simple and widely used method for obtaining information about solution concentrations.

Measurement Methods

• Relative density can be calculated by dividing the density of a sample by the density of the reference substance.
• Mass is divided by volume to calculate density.
• Irregularly shaped samples can be measured by displacement in a water-filled container.
• Surface tension of water may affect the accuracy of measurements.
• Hydrostatic weighing can be used to measure relative density by weighing the sample in air and water.
• Pycnometer is a device used to determine the density of a liquid.
• Hydrometer is a device used to measure the relative density of a liquid.

Other Concepts and Techniques

• True specific gravity is the ratio of the density of a sample to the density of water.
• Apparent specific gravity is the ratio of the weights of equal volumes of sample and water in air.
• Different industries use specific tables for relative density measurements.
• The pycnometer can also be used to determine the particle density of a powder.
• Digital density meters use various techniques such as hydrostatic pressure, vibrating element transducers, ultrasonic transducers, radiation-based gauges, and buoyant force transducers to measure density.
• Relative density is also used in soil mechanics to measure the current void ratio in relation to the maximum and minimum void ratios.

Relative density Data Sources